I. Studies on heavily exposed A-bomb survivors who have no clinical trouble at present ;a)Cytogenetic studies revealed 5- 45.6% of chromosome abnormalities in the peripheral lymphocytes, which were. estimated to be 9- 650 rad of exposure at the time of bombing.b)Serological studies for anti-BAST antibody shown that percent of position cases was significantly higher in the proximally exposed group.c)Molecular-biological studies revealed the transforming -N-ras and K-ras genes in two and one survivors, respectively.II. Studies on leukemia patients with a history of exposed to Atomic Bomb Radiation.a)Cytogenetic study on acute leukemia :Seventy five radiation--'related leukemia(acute non-lymphocyte)in Hiroshima including 16 patients exposed to more than one Gray were cytogenetically examined. Statistical analysis of the data on the frequencies of chromosomal aberrations in survivors according to the bone marrow doses of DS86 estimation revealed that heavily exposed patients tended to have significantly higher aberration rates as compared, -with non-exposed patients. Furthermore, the chromosomal aberrations in the survivors were-observed to be of a more complex nature and had characteristic findings of secondary leukemia.b)Molecular biological studies on transforming genes in acute and chronic leukemia and the bcr gene in chronic myelocytic leukemia have been performed in exposed and non-exposed groups. So far, no distinctive differences have been observed in the frequency and the sites of point mutations in N-. and K-ras genes or in the rearrangement of the bcr gene, for a final conclusion of the specificity of radiation induced leukemia.